In the long-slit or wide-mouth technique for extruding a film, foil, web or sheet of synthetic resin, the relatively wide and thin, generally noncoherent strip of molten thermoplastic material emerging from the wide mouth die at the end of the extruder must be cooled to form an extrusion with structural integrity.
One cooling technique makes use of a cooling roller or drum located immediately downstream from the mouth of the die and receiving the melt strip, which can also be referred to as a melt flag or melt banner, which passes onto the cooling surface of the roller which generally rotates at a peripheral speed such that it will match the rate at which the extrusion emerges from the die.
Because the melt strip has limited cohesion, it is important to ensure that it will adhere to the cooling roller or drum at least initially and for this purpose it has been proposed in practice to provide an electrostatic adhesion.
Juxtaposed with the drum is a wire corona-discharge electrode which generates an electrostatic field for this purpose. The cooling roller can be a counterelectrode for this field and is rotated with a predetermined angular velocity.
The corona electrode is so connected to the source of electric potential that it generates a corona discharge which, for example, can release electrons and the electrons can accumulate on the melt strip so that the latter is drawn against the positively charged counterelectrode, i.e. the cooling roller and is held against the surface thereof as the latter rotates and abstracts heat from the melt strip. When the melt strip has cooled sufficiently to form a foil or web, it can be peeled from the surface of the roller as a coherent and structurally stable member.
In the use of wire corona-discharge electrodes for such purposes for reasons which are not completely clear, visible stripes tend to be formed on the product, namely, the finished synthetic resin foil which affect the quality of the product.
Investigations have shown that in part these stripes may be due to differences in cooling effect across the width of the cooling drum or roller. For example, it is known in general that the cooling effect at the center of the cooling roller is greater than at the ends thereof.
However, these earlier associations of the cooling effectiveness with the stripes did not result in the development of a solution to the problem.
We have now found that the stripe-forming phenomenon may well be a result of the effect of the electrostatic fixing of the melt strip on the cooling roller surface.
In particular, we have discovered that the electrostatic adhesion of the melt strip to the cooling roller is not uniform over the entire width of the cooling roller. As a consequence, the heat transfer between the melt strip and the cooling roller is not uniform across the width of the latter and thus a visible stripe effect is observed.
We have found, moreover, that the variations in the electrostatic field adhesion of the melt strip to the cooling roller results from the corona-discharge electrodes which have been used. The corona-discharge wires, for example, may sag or deform so as to more closely approach the cooling roller at certain locations than at others. A contribution to this effect can result from temperature variations along the wire. Temperature differences can give rise to different electrostatic potentials so that there are different fields between the cooling electrode and the cooling wire at different points along the length of the corona wire.
The discharge may be more intensive in a sagging region at the middle of the corona electrode than at the ends thereof. Variations in the field generation resulting from variations in the thickness of the wire, simply because of manufacturing tolerances, can also contribute to the striping effect and the nonhomogeneous distribution of charge along the wire can also be a result of variations in surface roughness therealong.